1. Field of the Invention
The present invention relates to a MESFET (Metal Semiconductor Field-effect Transistor) and, more particularly to, a MESFET formed on a semi-insulative GaAs substrate.
2. Description of the Related Art
A GaAs MESFET is well-known device which is formed on a GaAs substrate which is electrically semi-insulative. FIG. 1 shows a cross-sectinal view of the GaAs MESFET.
As is shown in FIG. 1, this FET comprises GaAs substrate 11, N.sup.+ source region 12 formed in the surface region of substrate 11, N.sup.+ drain region 13 formed in the surface region of substrate 11, and N channel region 14 connecting regions 12 and 13. Source electrode 15, drain electrode 16, and gate electrode 17 are formed on GaAs substrate 11. Source electrode 15 and drain electrode 16 are in ohmic contact with N.sup.+ source region 12 and N.sup.+ drain region 13, respectively. Gate electrode 17 is in Schottky contact with N channel region 14. Depletion layer 101 is formed in that portion of channel region 14 which is located right below gate electrode 17. The depth and width of this depletion layer are determined by the reverse bias voltage (V.sub.G) applied to electrode 17. Depletion layer 102 is formed in the interface between GaAs substrate 11 and drain region 13.
Theoretically, a drain current I.sub.D of a certain value will flow in this GaAs MESFET when a predetermined DC bias voltage is applied between the gate and drain and also between the drain and source. Practically, however, the drain current fluctuates due to a excessive leak current I.sub.E (later described).
In FIG. 1, for simplicity of explanation, the arrows I.sub.D and I.sub.E represent the directions in which electrons move. The excessive leak current I.sub.E is generated as electrons or holes are generated in GaAs substrate 11. This excessive leak current I.sub.E has an AC component of 10 to 100 Hz.
Since drain region 13 has a potential higher than that of source region 12, the electrons generated in GaAs substrate 11 move into drain region 13 through depletion layer 102, in the form of excessive leak current I.sub.E. As the electrons move through depletion layer 102, the width of layer 102 changes. Thus, the width of the channel region 14 changes. As a result, the drain current I.sub.D is modulated; it fluctuates. The fluctuation of the drain current I.sub.D is discussed in Applied Physics Letter, 41(10), November 1982, p. 989, and Low Frequency Oscillation in GaAs IC's, 1985 IEEE GaAs IC Symposium.
The fluctuation of the drain current I.sub.D results in the fluctuation of the high-frequency gain of the MESFET. Consequently, the total gain of a device containing the MESFET will change. For example, a TV tuner containing the MESFET described above will cause flicker on a TV screen.